Asbestos and glass fibers were found to be 100 times more cytotoxic for human mesothelial cells and 10 times more cytotoxic for normal human bronchial epithelial cells than for normal bronchial fibroblasts. Chrysotile was the most cytotoxic fiber tested. Focal hyperplasia and epidermoid metaplasia were observed in explants of human bronchial tissue two weeks after a single exposure to amosite asbestos; both intracytoplasmic and intranuclear asbestos fibers were seen by X-ray microanalysis in these lesions. However, these atypical lesions proved to be transient and were not detected in tissue examined 12 weeks post exposure. Normal human bronchial cells were also exposed to amosite, but no abnormal cells were recovered from these cultures. On the other hand, colonies of phenotypically altered mesothelial cells arose in cultures two subculturings post amosite exposure. The control cultures ceased growth during the fourth subculture, whereas the amosite-exposed cultures continued to multiply for more than 18 subculturings. The exposed cells had a near-normal model number of chromosomes through the sixth subculturing. At the ninth subculture, 80% of the metaphases had dicentric chromosomes, and the model number had increased to 77.